Materials Science-Poland, 32(4), 2014, pp. 682-687
http://www.materialsscience.pwr.wroc.pl/
DOI: 10.2478/s13536-014-0250-9
Combustion synthesis process for the rapid preparation
of high-purity SrO powders
F RANCISCO G RANADOS -C ORREA∗ , J UAN B ONIFACIO -M ART ÍNEZ
Instituto Nacional de Investigaciones Nucleares, Departamento de Quı́mica, A.P. 18-1027. Col. Escandón, Delegación Miguel
Hidalgo. C.P. 11801 México, D.F., México
A rapid, safe and simple technique for the production of high purity strontium oxide powders via a chemical combustion
process is reported. The combustion reactions were performed to optimize the fuel to oxidizer ratios in the reaction mixtures
required to obtain pure SrO powders by varying the molar ratio of chemical precursors and the temperature. The synthesized
powders were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, infrared
spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and N2 -physisorption measurements. The results
indicate that crystalline SrO was obtained using a 1:1 strontium nitrate: urea molar ratio at 1000 °C after 5 minutes. In addition,
high-purity, homogeneous and crystalline SrO powders were easily produced in a short time via a chemical combustion process.
Keywords: strontium oxide; chemical combustion; XRD; IR spectral analysis; thermal analysis
© Wroclaw University of Technology.
1.
Introduction
In the past, the most extensive use of strontium
oxide powder (SrO) was in the cathode ray tubes
industry where it was employed in the form of an
aluminum alloy to help protect humans from X-ray
emissions in traditional color televisions. However,
this long-lasting technology has now been replaced
by the widespread use of flat displays (i.e. either
liquid crystal or plasma displays). Recently, SrO
powders have been essential in novel technological applications in the chemical and electronic industries, including the production of ferrite ceramic
magnets and zinc refining [1]. In addition, because
SrO is electronically conductive, it has been used
in fuel cells and oxygen generation systems as a
cathode material in the solid oxide form. Due to
its optical properties, strontium oxide has been employed to improve the quality of special glasses.
In addition, diverse strontium salts are currently
consumed as pyrotechnic materials or paint additives [2], or employed in fuel production catalysis [3]. Due to some unusual physical and chemical properties, strontium oxide has the potential for
use in medical appliances, such as tissue or body
∗ E-mail:
[email protected]
member replacements, restorative implant cement
or filling compounds [4].
Although the consumption demands fluctuate
from year to year, the overall consumption of strontium compounds and metals appears to be increasing [4]. Therefore, diverse technologies for the
synthesis of advanced ceramic materials, such as
strontium oxide, have been investigated to discover new cost effective and efficient methods of
production. It is important to note that nanocrystalline strontium oxide has been previously prepared using aerogel and conventional methods that
have yielded small amounts of product. In recent
years, combustion synthesis has become a promising technique because it is a fast and straightforward process that produces homogeneous, very
fine, and crystalline multicomponent oxide ceramic
powders [5, 6].
The chemical combustion synthesis is based on
the principle of explosive decomposition of nitrate
reagents and fuel mixtures using the instantaneous
heat generated by the chemical reaction between
the fuel and nitrates to convert the metal ions into
the target ceramic material [7]. The classical combustion synthesis consists in placing an aqueous
solution that contains salts of the metal of interest (the nitrates of the metal due to their good
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Combustion synthesis process for the rapid preparation of high-purity SrO powders
water solubility are typically preferred) in contact
with an appropriate organic fuel, such as urea, at
temperatures of 400 to 1100 °C. When the mixture begins to warm up, it generates instantaneous
heat that initiates a very fast and abrupt exothermal reaction that yields a metallic oxide in the
form of a fine powder, which is crystalline, amorphous, or semicrystalline and ready for use as advanced ceramics with interesting magnetic, dielectric, electrical, mechanical, luminescent, catalytic,
and optical properties [8–12]. The combustion process is characterized by high temperatures, fast
heating and short reaction times. These characteristics make the combustion method an attractive technique for the manufacture of technologically useful materials at a lower cost compared
to conventional ceramic processes where some require long heating times and equipment. Therefore, the metallic oxides obtained through chemical combustion provide an interesting alternative
compared to other complex synthesis processes because chemical combustion offers attractive advantages, such as simplicity as it requires little specialized equipment and enables rapid preparation
of a product with optimal structural and functional
properties. Typically, products of combustion synthesis are highly reactive and pure and contain
minimum level of impurities. Currently, several researchers have employed the combustion method
to synthesize a variety of oxide ceramic powders
with good success [13, 14]. In previous studies, we
have reported a successful synthesis of inorganic
materials via chemical combustion along with their
complete characterizations and demonstrated their
potential for application in environment remediation [15–18]. However, synthesis of SrO via chemical combustion has not been reported yet. Therefore, the aim of this study is to report the synthesis
of pure SrO powder using the chemical combustion
method. In addition, the particle structure and surface properties are discussed.
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used without further purification. Distilled water
was used to prepare the solutions, and the urea was
used as a fuel for the combustion reaction. The
chemical combustion of strontium nitrate and urea
as precursors was employed for the synthesis of
SrO.
2.2. SrO synthesis by chemical combustion
To establish the best experimental conditions of
SrO formation, suitable reaction parameters, such
as the molar ratio of chemical precursors (i.e. strontium nitrate and urea (1:1 to 1:2.5)) and the reaction temperature (i.e. 800, 900, and 1000 °C), were
tested to obtain a pure SrO powder. The appropriate
chemical precursors were transferred to a 50 mL
crucible, and the mixture was suspended in 1 mL of
distilled water until a homogeneous solutions was
obtained. Then, the mixture was heated until most
of the water evaporated with the aid of an electric
grill, resulting in a humid integrated solid. Next,
this solid was heated at the reaction temperatures
mentioned above for 5 min for the constant reaction time synthesis.
2.3.
Characterization
The synthesized powders were characterized
via various techniques. X-ray diffraction (XRD)
patterns were performed on a Siemens D5000 X-ray diffractometer with CuKα radiation.
Joint Committee on Powder Diffraction Standard
(JCPDS) file software was employed to determine the composition phases of the samples. Thermal analysis tests were performed using differential scanning calorimeter-thermogravimetric analysis (DSC-TGA) on a SDT Q600 calorimeter TA
Instrument-Waters equipment. The samples were
heated at a rate of 10 °C·min−1 under air flow of
100 mL·min−1 from room temperature to 1000 °C.
Infrared (IR) spectroscopy analysis was performed
using an IR Nicolet Magna 550 spectrophotometer
using a KBr disc. The microstructure and chemi2. Experimental
cal composition of the powders obtained by com2.1. Materials and methods
bustion were examined using a scanning electron
Strontium nitrate (Baker 98.5 % purity) and the microscopy (SEM) with energy dispersive X-ray
urea reagent (Sigma-Aldrich 99.9 % purity) were spectroscopy (EDS) analysis on a JEOL-JMS 5900.
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F. Granados-Correa, J. Bonifacio-Martı́nez
The Brunauer-Emmett-Teller (BET) specific surface area, mean pore diameter, total pore volume,
and the adsorption-desorption isotherm of the optimum synthesized material were determined by
N2 -physisorption measurements with a BelsorpMax Japan INC apparatus, and the samples were
degassed at 300 °C for 2 h prior to performing the
measurements.
3.
Results and discussion
3.1.
Synthesis of SrO
The effect of molar ratio of the chemical precursors and reaction temperature on the combustion
synthesis of SrO was investigated. First, the experiments were conducted using different molar ratios (i.e. 1:1, 1:1.5, 1:2.0, and 1:2.5) of the chemical precursors (i.e. strontium nitrate and urea) and
the combustion method described above at 800 °C,
900 °C and 1000 °C, in a muffle furnace under
atmospheric pressure with a constant reaction time
of 5 min.
The best conditions for SrO synthesis occurred
at a molar ratio of strontium nitrate to urea of 1:1
at 1000 °C for 5 min. This reaction involves the
exothermic reaction of an organic fuel (i.e., urea)
in the furnace chamber, which produces metal oxides via reduction of the metal nitrate [19]. Therefore, metal oxide products, gas emission of CO2 ,
H2 O, and nitrogen oxides (NO2 ) as well as other
products can be formed directly from the reaction
between metal nitrate and urea without using external oxygen. The resulting final products of rapid
combustion are very fine white powders, and the
balanced combustion reaction for the formation of
the SrO phase in this study can be written as:
2Sr(NO3 )2 + 3NH2 − CO − NH2 → 2SrO+
+ 2NO2 + 3CO2 + 3N2 + 2NH3 + 3H2 O
(1)
The presence of urea in the combustion reaction
produced SrO powders in a short reaction time (i.e.
5 minutes).
3.2.
3.2.1.
Characterization
X-ray diffraction study
Fig. 1 shows the X-ray diffraction patterns of
the SrO products synthesized by chemical combustion at a strontium nitrate to urea molar ratio of
1:1 heated at 800, 900 and 1000 °C for 5 minutes.
When the sample was calcined at 800 °C (sample SrO – A), the XRD pattern indicated the presence of three well-defined crystalline phases (i.e.
SrO, SrCO3 and Sr(OH)2 ) with SrO as the dominant phase (Fig. 1a). However, when the same sample was heated at 900 °C (sample SrO – B), the results in Fig. 1b, indicate that the sample primarily
consisted of SrO with minor intensities of SrCO3
and Sr(OH)2 impurities formed during the combustion reaction. For the sample heated at 1000 °C
(SrO – C), the results in Fig. 1c indicate the presence of only one crystalline phase corresponding to
the crystalline SrO phase in the final product, according to the JCPDS file No. 75-0263. The XRD
patterns (results no shown) of the samples obtained
at 800, 900 and 1000 °C with different molar ratios
of the chemical precursors (i.e. 1:1.5 to 1:2.5) were
analyzed. In these cases, a reaction temperature of
800 °C also resulted in the presence of three crystalline phases (i.e. SrO, SrCO3 and Sr(OH)2 ). However, a decrease in the relative peak intensity of
SrO was observed, and the relative intensities of the
peaks corresponding to SrCO3 and Sr(OH)2 phases
increased when the samples with molar ratios of
1:1.5 to 1:2.5 were heated at 900 °C. These results
suggest that the heat produced by the instantaneous
combustion of urea must be the principal heat
source for SrO production and that an increased
amount of urea (>1.5) favored the production of
the strontium carbonate and strontium hydroxide
phases. Therefore, the differences observed in the
synthesis of SrO were due to the stoichiometric
amount of fuel (urea), and the urea combustion was
shown to be an important factor initiating SrO synthesis, which has been observed by Cruz et al. [20]
for the synthesis of Li2 MO3 (M = Ti or Zr) using
a combustion method where a stoichiometric relationship of the chemical precursors was determined
to be 1:1. Based on these results, well-crystallized
SrO powder can be easily obtained under the
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Combustion synthesis process for the rapid preparation of high-purity SrO powders
685
experimental conditions described above (i.e. an oxidant. Next, two sharp endothermic peaks are
strontium nitrate to urea molar ratio of 1:1 at observed at approximately 617.5 and 668.4 °C due
to the phase formation of SrO, as shown in Fig. 2.
1000 °C for 5 min).
This behavior indicates that the formation reaction
is very fast and explosive above 600 °C, and the end
product is stable from 740 °C to 1015 °C. Therefore, the succeeding horizontal line is due to SrO.
TGA-DSC studies confirm the phase formation of
SrO during the heating process, which is consistent
with the behavior reported by Cambpell et al. [21].
Cambpell et al. determined that the formation of
SrO by the calcination of strontium nitrate, started
at 600 °C and was completed at 745 °C. In addition, the reported end product was SrO, which was
stable up to 1000 °C. In our case, the end product
was confirmed by XRD analysis.
Fig. 1. XRD patterns for the SrO powders prepared
using combustion method at a molar ratio
of strontium nitrate to urea 1:1, at different
temperatures, for 5 min.
3.2.2. Thermal analysis study
The thermal studies of the sample were carried
out at a heating rate of 10 °C/min up to 1000 °C
for 5 minutes, and the TGA/DSC curves are shown
in Fig. 2. It can be observed that the thermal decomposition process proceeds in four stages. The
first step, corresponds to the dehydration of the hydroxyl water of SrOOH, which is transformed to
SrO resulting in a weight loss of 2.19 %. The second mass loss corresponds to the release of cyanic
acid and other volatile product, such as NO2 , with a
weight loss of 2.48 %. The third step corresponds to
the release of nitrous fumes and CO2 with a weight
loss of 47.58 %. The final step is due to the crystallization of SrO with a weight loss of 1.63 %. The
net weight loss of the compound was determined to
be 53.88 %. In the DSC curves there are endothermic peaks below 291.9 °C caused by the vaporization of physically bound absorbed water and dehydration of the powders. In addition, an exothermic peak, which corresponds to decomposition can
be observed at 495 °C. Such an exothermic reaction is a result of a thermally induced redox reaction where urea acts as a reductant and nitrate as
Fig. 2. DSC-TGA curves of SrO prepared by the combustion method at 1000 °C for 5 min.
3.2.3. Infrared spectral study
The infrared spectrum of the sample obtained
at urea to strontium nitrate molar ratio of 1:1 at
1000 °C (sample SrO – C) is shown in Fig. 3. The
IR bands of SrO are shown over the wavelength
range of 4000 to 400 cm−1 and the broad band at
3497 cm−1 corresponds to the OH−1 stretching vibration due to hydroxyl functional groups as well
as hydrogen bonding. The peaks at 2357 cm−1 and
2341 cm−1 indicate the presence of K–Br stretching. The band at 1709 cm−1 caused by the presence
of conjugated C–O, which is most likely related
to the presence of CO2 due to the IR study performed under uncontrolled atmospheric conditions.
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686
F. Granados-Correa, J. Bonifacio-Martı́nez
The band at 1444 cm−1 may be due to bending vibrations from O–H groups. The band at 1036 cm−1
can be assigned to Sr–O–Sr stretching vibrations.
Therefore, the IR spectra confirm the formation of
SrO with hydroxyl groups due to the presence of
surface water and do not exhibit bands corresponding to organic residues. Similar observations have
been reported in the literature for the synthesis of
Ni powder by microwave-assisted combustion using urea as a fuel [22].
Fig. 4. SEM image and EDS analysis of a SrO sample
prepared by chemical combustion at 1000 °C for
5 min.
diameter is 29.41 nm. Fig. 5 shows the N2
adsorption-desorption isotherm obtained for the
synthesized compound (i.e. sample SrO – C). According to the International Union of Pure and Applied Chemistry (IUPAC), SrO exhibits an isotherm
type II, which is typically associated with nonporous materials, as observed in our SEM study.
The adsorption of gases and vapors give rise to
I – IV types of adsorption isotherms with differFig. 3. IR spectrum of SrO synthesized by chemical
ent characteristics, which are primarily dependent
combustion at 1000 °C for 5 min.
on the structure of the solids. Type II isotherms,
which are observed in the physical adsorption of
gases by non-porous solids, are multilayered and
3.2.4. SEM-EDS study
The morphology of the as-synthesized material often referred to as sigmoid isotherms [23].
(sample SrO – C) is shown in Fig. 4. As shown in
Fig. 4, the combustion reaction product (i.e. SrO) is 4. Conclusions
observed as square lamina-like aggregates consistStrontium oxide was synthesized using a coming of particles with smooth surfaces. In addition,
the SEM micrograph of the SrO – C sample shows bustion method, and the results from several anavarious SrO particles that are 1 to 30 µm in size lytical tests indicate that the prepared material conafter 5 min of chemical combustion at 1000 °C. sists of a highly crystalline phase. The procedure
The EDS analysis in Fig. 4 confirms that the SrO involved using a molar ratio of strontium nitrate to
particles consist only of strontium and oxygen urea 1:1 and heating at 1000 °C for 5 min. This
procedure represents a novel method for producelements.
ing a useful oxide ceramic material. The use of a
3.2.5. N2 -physisorption measurements
combustion method to obtain this ceramic material
N2 -physisorption measurements indicate that resulted in the production of an advanced material.
the BET specific surface area value for SrO (sam- In addition, no more than 5 min were required for
ple SrO – C) is 4.45 m2 /g, the total pore vol- the synthesis reaction to obtain the final pure SrO
ume is 8.10 × 10−3 cm3 /g, and the mean pore powders.
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687
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Acknowledgements
The authors wish to thank ININ, México, for financial
support through Department Activity No. DIO2B. The authors
also wish to thanks Jorge Cid del Prado and Isidoro Martı́nezMera for providing the XRD, SEM, and EDS facilities.
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Received 2014-06-20
Accepted 2014-09-03
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